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Fungal colonisation strategies Pt. III: Specialised opportunism

This strategy sees fungi colonise the sapwood in a living tree by taking advantage of the tree’s physiological stress due, for example, to root dysfunction or drought conditions (Boddy, 2001; Parfitt et al., 2010; Rayner, 1993; Rayner & Boddy, 1988; Schwarze, 2008). Development will be in apparently intact, yet dysfunctional sapwood of areas of the tree which remain uninjured, though decay onset will be timed with desirable conditions within the tree (induced by stress). Single genotypes will usually manifest with distinct speed (up to a few metres per year), and spread extensively, using the xylem as a vector, throughout underlying sections of bark, forming vast decay columns. This therefore entails that such decay fungi are present extensively within the tree (yet not in an overt manner) within its functional sapwood, prior to attack. Onset of decay is likely not observed until the tree suffers localised xylem dysfunction, given the high water content of functional sapwood is undesirable for fungal decay (Baum et al., 2003; Boddy, 2001).

This mature Betula pendula, whilst still alive, has been aggressively colonised by Piptoporus betulinus, following physiological stress.

Latent invasion may also in fact result from the development and subsequent assimilation of separate fungal mycelia, under the conditions associated with dysfunction. The spores may have spread widely within the sap stream over long periods, initiating only later (following stress in the host) their mycelial development, with subsequent ‘assimilation’ of the many establishing mycelium networks as they coalesce. The consequent decay associated with the assimilation and the host’s inability to defend against the widespread attack by the host may ultimately be very significant (Boddy & Rayner, 1983; Parfitt et al., 2010). Research also suggests that even once sapwood does become dysfunctional, presence of decay may not become overt. Decay may not even begin whatsoever. Further, as many fungal species latently exist within specific hosts, particular conditions may only trigger the onset of decay by one, or a portion of, the fungal species present (Parfitt et al., 2010).

Additionally, such strategists have a high degree of selectivity with regards to their host site and / or species, with branch junctions being a principal location for decay onset (Boddy, 2001; Rayner, 1993). This is perhaps due to the lower side of the branch junction being an inherent weak point within the tree, because the site has low energy reserves – particularly when the branch attached to the parent branch or trunk is dying (Shigo, 1986). An example of a specialised opportunist’s strategy is therefore the entering into a dying branch with sapwood dysfunction, likely induced by the inability to compete with its neighbours for light, waiting at the junction of the dying branch until the spores are incorporated into the heartwood via secondary thickening, and then establishing and beginning the attack (Baum et al., 2003; Chapela & Boddy, 1988a; Chapela & Boddy, 1988b; Oses et al., 2008). Such a colonisation trait can be described as endophytic – this is where a species resides within the host with no adverse impact upon the host until conditions are right for attack (Baum et al., 2003). Such a scenario may even be beneficial in terms of facilitating the “natural pruning” of limbs that become dysfunctional as tree canopies expand (Rayner, 1993).

Under some conditions, certain specialised opportunists may also be able to colonise via active pathogenesis (Rayner, 1993).